This invention relates generally to optical communication systems and components. More particularly, the invention relates to optical add-drop multiplexers (OADM) comprising micro- and nano-scale optical structures and components built from and upon monolithic substrates.
In the field of telecommunication, it is recognized that optical communication components and systems, which use light waves and beams to carry information, offer many considerable advantages over conventional copper wire-based communication systems that carry information in the form of electrical signals. One advantage is the significantly greater amount of information that can be carried by a physical connection employing a single fiber optic strand as compared to a copper wire circuit.
To optimize the amount of information that can be transmitted along a single optical fiber, the technique known as Dense Wavelength Division Multiplexing (DWDM) is now being implemented. The use of DWDM is accelerating at a rapid pace, due to the development of essential network components such as optical fiber, infrared laser transmitters, fiber amplifiers, and the like. However, the rate of growth of DWDM networks is currently limited in part by the availability of low-cost mass produced components that provide acceptable reliability and resistance to environmental effects, such as, vibration, mechanical stresses, and temperature fluctuations.
In these optical telecommunications systems, information is transmitted in the form of infrared light signals that originate at laser sources. Each laser source is tuned to emit an infrared light beam comprising a narrow band of wavelengths (or, equivalently, frequencies) centered at a primary frequency. As used herein, the word xe2x80x9ccolorxe2x80x9d describes a characteristic band of wavelengths centered around a specific primary wavelength as emitted by a telecommunications laser. Information is encoded in each infrared beam by temporally modulating the laser power. Each primary frequency corresponds to a value specified by the standard International Telecommunication Union (ITU) grid. The ITU standard specifies transmission frequencies that are spaced at 100 GHz intervals, although a reduction to 50 or 25 GHz is anticipated as the technology evolves.
In DWDM, many distinct colors of infrared light may be transmitted simultaneously along a single optical fiber, each color carrying information that is distinct from information carried by other colors. Devices called multiplexers physically superimpose the light beams from several colors thereby creating multi-color light beams wherein each color carries its encoded information. The combined information is transmitted some distance. At a terminus of the transmission path, demultiplexers physically separate or disperse the multiple colors received from a single optical fiber onto multiple output fibers, each output fiber carrying a single color, thereby permitting the information carried by each color to be directed to its intended destination. An optical add-drop multiplexer removes light of a particular color from a polychromatic beam, and replaces the color removed with a beam having substantially the same color. This process of removing and replacing signals corresponding to a specific color provides the capability to switch signals into and out of optical beams.
The ideal demultiplexer will direct all of the incoming light of each color onto its corresponding output optical fiber. However, in actual demultiplexer devices the color separation is generally imperfectxe2x80x94not all of the light of each color entering the demultiplexer is transmitted into each respective output beam, and a portion of the light from each color is transmitted into the paths of neighboring beams. This leakage causes undesirable performance effects such as crosstalk and insertion loss that must be limited in magnitude for the device to be practical.
At present, demultiplexing is often accomplished utilizing devices based upon either diffraction gratings or precision interference filters. Neither type of device is amenable to cost-effective mass production while maintaining acceptable performance.
For filter-based demultiplexers, such as those described in T. E. Stem, K. Bola, Multiwavelength Optical Networks, A Layered Approach, Addison Wesley, 1999, each filter must be manufactured separately from the others using multilayer vapor deposition techniques. The filters are then installed manually or robotically in an optical substrate and aligned to project light onto individual output optical fibers. Achieving and maintaining optical alignment in spite of thermal and mechanical stresses confounds attempts to reliably mass produce these devices, adding to the production cost and diminishing long-term reliability and resistance to environmental influences. Furthermore, the filters are frequently operated in a serial configuration, such that one color is transmitted through one filter while all other colors are reflected to the next, and so on. In this configuration, the insertion loss accumulates so that the last transmitted color has significantly higher loss than earlier colors.
Grating-based devices offer the advantage of being parallel rather than serial demultiplexers, and therefore have improved insertion loss uniformity. However, to achieve acceptable crosstalk, the optical components within grating-based devices must be several centimeters in size. Therefore, like filter-based devices, grating-based demultiplexers are difficult to align and maintain aligned. Low-cost mass production of reliable devices remains elusively difficult.
Recently, demultiplexers based on arrayed waveguide gratings (AWGs) have been introduced commercially. These small, thin monolithic devices, generally fabricated from silicon-based or InP-based wafer substrates, offer promise as low-cost components that can be mass produced. Nevertheless, despite more than a decade of intense development of AWG technology, and the emergence of several companies offering AWG products, the performance specifications achieved by production AWGs remain several orders of magnitude worse than theoretical predictions.
Monolithic demultiplexer devices based on waveguide gratings etched into wafers of semiconductor materials such as InP have been described in recent patent and technical literature. These devices also offer potential as low-cost mass-producible components but, like AWGs, have not yet achieved acceptable performance specifications. In particular, etched waveguide gratings demonstrated to date suffer from excessive crosstalk due to the small size and imperfections in fabrication of the grating structure.
The invention, in one embodiment, provides a miniature monolithic optical wavelength add-drop multiplexer comprising an assembly of optical components built on a monolithic platform.
In one aspect the invention relates to a miniature monolithic optical add-drop multiplexer. The miniature monolithic optical add-drop multiplexer includes a monolithic substrate, a wavelength dispersive optical element fabricated on the monolithic substrate, a wavelength filter array fabricated on the monolithic substrate, and a diverter. The wavelength dispersive optical element receives an input beam having a plurality of spatially overlapping distinct colors and providing an output signal composed of a plurality of spatially separated substantially single-color beams, each substantially single-color beam having a primary wavelength that is different than the primary wavelengths of the other substantially single-color beams. The wavelength filter array fabricated on the monolithic substrate has at least one filter element. The at least one filter element receives a selected one of the plurality of spatially separated substantially single-color optical beams and removes therefrom any portions of beams of other primary wavelengths that were separated incompletely from the selected beam by the wavelength dispersive optical element, thereby providing a purified single-color output beam substantially free of colors associated with other primary wavelengths and having a first direction of propagation. The diverter intercepts the purified single-color output beam and diverts the output beam from the first direction of propagation.
In one embodiment, the substrate comprises at least one material selected from the group consisting of silicon, silicon monoxide, silicon dioxide, silicon-germanium alloys, silicon carbide, silicon nitride, and indium phosphide. In one embodiment, the substrate comprises a material that is amenable to processing using semiconductor fabrication processes. In one embodiment, at least one of the wavelength dispersive optical element, the wavelength filter array, and the diverter comprises at least one material selected from the group consisting of silicon, silicon monoxide, silicon dioxide, silicon-germanium alloys, silicon carbide, silicon nitride, and indium phosphide.
In one embodiment, at least one of the wavelength dispersive optical element and the wavelength filter array elements comprises an optical waveguide. In one embodiment, at least one of the wavelength dispersive optical element and the wavelength filter array elements comprises a miniature free-space optical element.
In one embodiment, the miniature monolithic optical add-drop multiplexer further comprises an input optical structure that receives the input beam. In one embodiment, the input optical structure comprises at least one material selected from the group consisting of silicon, silicon monoxide, silicon dioxide, silicon-germanium alloys, silicon carbide, silicon nitride, and indium phosphide. In one embodiment, the input optical structure comprises an optical waveguide. In one embodiment, the input optical structure comprises a miniature free-space optical element. In one embodiment, the miniature monolithic optical add-drop multiplexer further comprises an optical waveguide that communicates the input beam from an external source.
In one embodiment, the miniature monolithic optical add-drop multiplexer further comprises an array of output optical structures having at least one output element, the at least one output element transmitting an output beam. In one embodiment, the array of output optical structures comprise at least one material selected from the group consisting of silicon, silicon monoxide, silicon dioxide, silicon-germanium alloys, silicon carbide, silicon nitride, and indium phosphide. In one embodiment, the array of output optical structures comprise optical waveguides. In one embodiment, the array of output optical structures comprise miniature free-space optical elements.
In one embodiment, the miniature monolithic optical add-drop multiplexer further comprises an optical waveguide that communicates the output beam from the output element to an external receiver. In one embodiment, each color comprises a narrow band of wavelengths centered on a primary wavelength. In one embodiment, each primary wavelength is designated by the International Telecommunications Union as one of a set of discrete wavelengths to be utilized for optical telecommunications. In one embodiment, the wavelength dispersive optical element is a selected one of a prism, a grating, and a grism. In one embodiment, the wavelength filter array is a selected one of an array of interference filters, an array of waveguide Bragg gratings, an array of Fabry-Perot interferometers, an array of resonantly-coupled waveguide structures, and an array of waveguide ring resonators.
In one embodiment, the diverter is fabricated on the miniature monolithic substrate. In one embodiment, the diverter comprises a reflective surface. In one embodiment, the diverter comprises an electromechanical actuator that can move the reflective surface. In one embodiment, the diverter comprises an optical resonator. In one embodiment, the diverter comprises an electrically-driven actuator that can alter a resonant frequency of the optical resonator. In one embodiment, the diverter is capable of being dynamically reconfigured to divert a selected one of the plurality of purified single color output beams.
In one embodiment, the miniature monolithic optical add-drop multiplexer further comprises a second diverter adjacent to the miniature monolithic optical demultiplexer, and optionally, a second wavelength dispersive optical element. The second diverter receives a second input beam having a purified single color substantially identical to the purified single color of the output beam that is diverted. The optional second wavelength dispersive optical element is capable of combining a plurality of spatially-separated substantially single color optical beams, at least one beam containing primarily a single color that is different than the color of another substantially single color optical beam, into a single beam having a plurality of spatially overlapping distinct colors. The second diverter is adapted to direct the second input beam for combination with another spatially-separated substantially single color optical beam having a color distinct from that of the second input beam to form a second output beam having a plurality of spatially overlapping distinct colors, using a selected one of the wavelength dispersive optical element and the optionally fabricated second wavelength dispersive optical element to effect the combination.
In another aspect, the invention features a method of processing an optical beam having a plurality of spatially overlapping colors. The method comprises the steps of providing an assembly of miniature optical elements, receiving an input beam having a plurality of spatially overlapping distinct colors, dispersing the input beam into a plurality of spatially-separated substantially single color optical beams, each beam containing primarily a single color that is different than the color of the other substantially single color optical beams, filtering the at least one of the substantially single-color optical beams and removing therefrom colors other than the designated primary color of that beam, thereby providing at least one purified single-color output beam having a first direction of propagation, the output beam being substantially free of other colors, intercepting the at least one purified single-color output beam and diverting the output beam from the first direction of propagation. The assembly of miniature optical elements comprises a monolithic substrate, a wavelength dispersive optical element fabricated on the monolithic substrate, a wavelength filter array fabricated on the monolithic substrate, and a diverter. The input beam is dispersed by use of the wavelength dispersive optical element.
In one embodiment, the input beam is received from an optical waveguide external to the assembly. In one embodiment, the output beam is transmitted to an optical waveguide external to the assembly.
In one embodiment, the method further comprises providing a second assembly of miniature optical elements, receiving a second input beam having a purified single color substantially identical to the purified single color of the output beam that is diverted, controlling a direction of propagation of the second input beam, and directing the diverted second input beam and another spatially-separated substantially single color optical beam having a color distinct from that of the diverted second input beam to form a second output beam having a plurality of spatially overlapping distinct colors. The second assembly of miniature optical elements comprises a second monolithic substrate, a second wavelength dispersive optical element fabricated on the monolithic substrate, the second wavelength dispersive optical element capable of combining a plurality of spatially-separated substantially single color optical beams, at least one beam containing primarily a single color that is different than the color of another substantially single color optical beam, into a single beam having a plurality of spatially overlapping distinct colors, and a second diverter. The direction of propagation of the second input beam is controlled with the second diverter. The second wavelength dispersive optical element is used to combine the second input beam and another spatially-separated substantially single color optical beam.
In one embodiment, the method further comprises transmitting the second output beam having a plurality of spatially overlapping distinct colors to an optical waveguide external to the second assembly. In one embodiment, the monolithic substrate and the second monolithic substrate are the same monolithic substrate. In one embodiment, the wavelength dispersive optical element and the second wavelength dispersive optical element are the same wavelength dispersive optical element. In one embodiment, the second input beam is the diverted purified single-color output beam. In one embodiment, the second input beam is substantially a duplicate of the diverted purified single color output beam. In one embodiment, the second input beam is a modified version of the diverted purified single color output beam.
In yet another aspect, the invention relates to a method of fabricating a miniature monolithic optical add-drop multiplexer. The method comprises the steps of utilizing semiconductor fabrication methods to create a wavelength dispersive optical element and an array of wavelength filtering elements upon a monolithic substrate, the combination of the wavelength dispersive optical element and the array of wavelength filtering elements providing a miniature monolithic optical demultiplexer, and providing a diverter adjacent to the miniature monolithic optical demultiplexer, the diverter adapted to intercept and divert an optical beam that passes through the miniature monolithic optical demultiplexer.
In one embodiment, the wavelength dispersive optical element and the wavelength filter array are formed by semiconductor fabrication processes. In one embodiment, the diverter is formed by semiconductor fabrication processes. In one embodiment, a diverter component is formed on the monolithic substrate.
In one embodiment, the fabrication method further comprises the step of fabricating an input optical structure by semiconductor fabrication processes. In one embodiment, the fabrication method further comprises the step of fabricating an output optical structure by semiconductor fabrication processes. In one embodiment, the fabrication method further comprises the steps of providing a second diverter adjacent to the miniature monolithic optical demultiplexer, the diverter receiving a second input beam having a purified single color substantially identical to the purified single color of the output beam that is diverted, and optionally fabricating, utilizing semiconductor fabrication methods, a second wavelength dispersive optical element, the second wavelength dispersive optical element capable of combining a plurality of spatially-separated substantially single color optical beams, at least one beam containing primarily a single color that is different than the color of another substantially single color optical beam, into a single beam having a plurality of spatially overlapping distinct colors. The second diverter is adapted to direct the second input beam for combination with another spatially-separated substantially single color optical beam having a color distinct from that of the second input beam to form a second output beam having a plurality of spatially overlapping distinct colors, using a selected one of the wavelength dispersive optical element and the optionally fabricated second wavelength dispersive optical element to effect the combination.
The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims.